The present invention relates to a controllable change-over apparatus for a solar module and a method for controlling the same.
A solar module converts electromagnetical radiation, such as sunlight, into electric energy and comprises, for example, several solar cells.
Solar modules are connected individually or in groups, for example, in photovoltaic plants, small power-supply independent users, or for power supply of spacecrafts.
The electric power ratings of a solar module characterizing a solar module are, for example, open-circuit voltage and short-circuit current. The same can depend on the properties of the individual solar cells and the connection of the solar cells within the module.
When several modules are operated in series, a freewheeling diode or a bypass diode, respectively, can be connected anti-parallel to every module or partial string, respectively. The freewheeling diode can be connected to the output terminals of every module such that the same is reverse-biased in the normal operating state when the module provides current.
Normally, solar modules are provided with a connection box housing the bypass diodes connected anti-parallel to the respective partial strings in the module and the output terminals for the solar module.
If a partial string of the module is shaded, the bypass diode of the same will conduct the differential current between the current of the shaded module string and the solar generator string. Due to the fact that the bypass diode is conductive, the voltage of the shaded module string becomes almost zero (approximately −1 Volt), such that the shaded module string does no longer produce any power.
For avoiding this, DC/DC converters can be used, adapting current and voltage such that the shaded module string still dissipates power. It is, however, a disadvantage that the DC/DC converter also operates in the non-shaded case and, produces electrical losses and reduces the overall efficiency of the solar generator.
FIG. 7A shows the case of a solar module 101 directly coupled to the module terminal 102, while FIG. 7B shows the case of a solar module 101 connected to the module terminal 102 via a DC/DC converter 105. In the case of FIG. 7Aa, no power losses occur due to a DC/DC converter 105, but shadings of the solar module 101 lead to the above-described problems. The other way round, in the case of FIG. 7B, shading of the solar module 101 does not cause an interruption of the power dissipation, but the overall efficiency of the solar generator is affected, even with full illumination of the solar module 101.